Cobalt-base tantalum carbide eutectic alloys

ABSTRACT

Unidirectionally solidified cobalt-base carbide-reinforced cast superalloy bodies having high strength and high resistance to creep, particularly at elevated temperatures, and having an approximate nominal (melt) composition range which consists essentially, by weight, of a trace to 26.0 percent chromium, 13.5 to 19.0 percent tantalum, a trace to 14.5 percent nickel, a trace to 6.5 percent tungsten, a trace to 1.0 percent iron and 1.2 to 1.4 percent carbon, balance essentially cobalt. A preferred composition contains the nominal (melt) percentages: 15.0 w/o chromium, 8.5 w/o nickel, 6.0 w/o tungsten, 18.67 w/o total tantalum, 1.33 w/o carbon, for a total of 20.0 w/o TaC. The resulting article, after trimming, contains from 11.5 to 12.5 wt. percent tantalum. The tantalum and carbon, primarily as an aligned dispersion of TaC fibers in the matrix of the composite structure, constitutes about 5 to 15 volume percent of the body.

[ Feb. 19, 1974 COBALT-BASE. TANTALUM CARBIDE EUTECTIC ALLOYS Inventors:John L. Walter, Scotia; Harvey E.

Cline, Schenectady, both of N.Y.

Assignee: General Electric Company,

Schenectady, N.Y.

Filed: Sept. 21, 1971 Appl. No.: 182,530

Related US. Application Data [63] Continuation-impart of Ser. No.134,235, April 15,

1971, abandoned.

[52] US. Cl. 75/171, 148/32 [51] Int. Cl. C22c 19/00 [58] Field ofSearch..... 75/134 F, 170, 171; 148/32, 148/325 [5 6] References CitedUNITED STATES PATENTS 3,528,808 9/1970 Lemkey et al. 75/170 3,260,5057/l966 Ver Snyder 75/171 3,085,005 4/1963 Michael et al. 75/171 PrimaryExaminer-Richard 0. Dean Attorney, Agent, or FirmGerhard K. Adam; JosephT. Cohen; Jerome C. Squillaro [5 7] ABSTRACT Unidirectionally solidifiedcobalt-base carbidereinforcd cast superalloy bodies having high strengthand high resistance to creep, particularly at elevated temperatures, andhaving an approximate nominal (melt) composition range which consistsessentially,- by weight, of a trace to 26.0 percentchromium, 13.5 to19.0 percent tantalum, a trace to 14.5. percent nickel, a trace to 6.5percent tungsten, a'trace to 1.0 percent iron and 1.2 to 1.4 percentcarbon, balance essentially cobalt. A preferred composition contains thenominallmelt) percentages: 15.0 w/o chromium, 8.5 w/o nickel, 6.0 w/otungsten, 18.67 w/o total tantalum, 1.33 w/o carbon, forla total of 20.0w/o' TaC.

The resulting article, after trimming, contains from 11.5 to 12.5 wt.percent tantalum. The tantalum and carbon, primarily as an aligneddispersion of TaC fibers in the matrix of the composite structure,constitutes about 5 to 15 volume percent of the body.

2 Claims, 3 Drawing Figures Carbon- COBALT-BASE TANTALUM CARBIDEEUTECTIC ALLOYS CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OFTHE INVENTION 1. Field of the Invention The present invention relates ingeneral to cobaltbase eutectic superalloy compositions, and moreparticularly to bodies of such compositions containing, asreinforcements of their structure, aligned fibers of a carbide.

In particular embodiments of this invention, these a1- loys have utilityin gas turbine engine components.

2. Description of the Prior Art Superalloy bodies forming components ingas turbine engines are often subjected to temperatures which canapproach 1,900F, under highly oxidizing conditions. Althoughunidirectional solidification of a number of eutectic superalloys isknown to achieve increased strength, heretofore there have been nocobalt-base superalloy compositions developed which can beunidirectionally solidified to have a good combination of properties ofhigh strength, high resistance to creep or stress-rupture at elevatedtemperature and good resistance to high temperature oxidation,a1l whileretaining adequate ductility.

Among the commercially available cast cobalt-base superalloys used forgas turbine engine blades is the following:

Cobalt balance Chromium 25.0 Nickel 10.0 Tungsten 7.5 Iron I .0

However, the stress-rupture properties of this and similar alloys athigh temperatures leave much to be desired.

SUMMARY OF THE INVENTION lidified superalloys which have excellenttensile properties, at room temperature and at temperatures up to2,000F.

Another object of the invention is to provide cast fiber-strengthenedbodies useful as components for turias 2 .24392 wb qhhave s b a o of Pri including high yield and tensile strength combined with good creepresistance under static stress at elevated temperatures, together withgood high-temperature oxidation resistance, with good engineeringductility.

Still another object of the invention is to provide a cast articlehaving a microstructure which includes a relatively high volume percentof aligned reinforcing fibers in the matrix of the alloy.

Other objects and advantages of the invention will become apparent fromthe following description and appended claims.

In accordance with the objects of the invention, a range of cobalt-basesuperalloy compositions is provided which form a composite structurecomprising a matrix reinforced with aligned fibers essentially oftantalum carbide. The range of nominal (melt) compositions of theunidirectionally solidified castings, according to the invention isapproximately, in weight percent, a trace to 26.0 percent chromium, 13.5to 19.0 percent tantalum, a trace to 10.0 percent nickel, a trace to 6.5percent tungsten, a trace to 1.0 percent iron and 1.2 to 1.5 percentcarbon, balance essentially cobalt.

DESCRIPTION OF PREFERRED EMBODIMENTS form TaC, which makes up about 12volume percent of V the alloy.

It is therefore an object of the present invention to' The resultingarticles after trimming off redundant portions not having the uniformlyaligned TaC fibers,

have the preferred composition range: 14.0 to 18.0 w/o Cr, 11.5 to 12.5w/o Ta, 8.0 to 10.0 w/o nickel, 6.0 to 8.5 w/o W, and 0.8 to 1.5 w/ocarbon, balance substantially cobalt.

In order to obtain the most desirable metallographic structures andproperties disclosed herein, the alloys were unidirectionally cast at asolidification rate of A inch/hr. in a temperature gradient of about 250per inch.

The alloys of the present invention contain carbide fibers, essentiallyof tantalum carbide in amounts of from 5 to 15 volume percent of thealloy, with the preferred composition containing at least about 12volume percent. The tantalum carbide fibers are preferably dispersedthroughout the composite structure, but beyond about 12 volume percentthe carbide tends to precipitate out as particles at the bottom of thecast body.

Typical compositions according to the invention are sho n bel w in TableI.

TABLE I Cobalt Base Carbide Alloys Composition -wt.

Designation Co Cr Ni W Ta C Fe Ta(M)* A) TaC-C045 I 45.0 25.5 8.5 6.013.8 |.2 B) 'luC-Co50 50.5 20.0 11.5 6.0 3.8 1.2 c) TaC-C056 56.5 20.014.5 13. 1.2

I I Continued i Cobalt Base Carbide Alloys Composition wt.

Designation Co Cr Ni W Ta C Fe Ta(M)' D) TaC-C051 51.5 20.0 8.5 18.671.33 E) TaC'Co55 55.5 15.0 8.5 6.0 13.8 1.2 F) TaC-CoSOB 50.5 15.0 8.56.0 18.67 1.33 G) TaC-CoSUTa 50.0 17.5 8.5 6.0 13.8 1.2 3 H) TaC-C04949.04 20.0 8.5 6.4 13.8 1.2 0.85 I) TaC-C050Ni 50.5 15.0 14.5 0.0 18.671.33 F-l) (Article 54.8 16.4 9.3 8.2 12.2 0.8

portion) of TaC.

The composition designated in Table I as TaC-CoSOB, corresponding tomelt composition F and article portion composition F-l (after trimming)is a preferred embodiment of the invention because of its excellentproperties, as shown in Tables 11, Ill and IV. The tantalum carbide inthe melt composition of this alloy is 20.0 wt. percent.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be betterunderstood from the following description, taken in conjunction with theac companying drawings, wherein:

FIG. 1 is a photomicrograph (250X) of a longitudinal section of aspecimen of composition designated TaC- 50B, according to a preferredembodiment of the invention;

FIG. 2 is a photomicrograph (250X) of a transverse section of a specimencorresponding to that of FIG. 1; and

FIG. 3 is a chart illustrating stress-rupture properties under stress athigh temperature, as compared to a known superalloy, Ren 80. plottingthe life at temperature as the Larson-Miller parameter vs. stress.

In order to illustrate more completely the substantial 'Ta(M) refers tothe wt. percent of tantalum in the matrix, as distinguished from and inaddition to the tantalum present in stoichiometric proportions withcarbon as fibers improvements in physical properties which are achievedby the alloys of the present invention, Tables 11 and III belowrespectively list the tensile properties at room temperature and atelevated temperature of 1,832F (1,000C). Table IV lists the hightemperature stress-rupture properties of the alloys at the variousdesignated temperatures.

EXAMPLE I lngots of the melt composition designated in Table l as F),TaC-CoB, were prepared and directionally solidified from moltencondition at 1 1 inch/hr. and in a temperature gradient of about250C/inch. After customary trimming off of redundant segregatedportions. the resulting microstructure, as may be seen in FIGS. 1 and 2,comprised aligned carbide fibers essentially single crystal tantalumcarbide, in a cobalt-base alloy matrix. The mechanical properties of theuniform fibrous portion of the preferred alloy TaC-CoSOB, had thearticle composition designated F1 in Table I. The results of specimensof each of the alloys of Table I, tested at a strain rate of 2 X l0'/min. are listed in Table II for room temperature and in Tables Ill andIV at the elevated temperature of 1,832F.

. BLE 11 Room Temperature Tensile Properties Ult. Stress CompositionDescription (psi) elong.

A) TaC-C045 Cobalt superalloy with TaC 1 11,300 1.0 B) TaC-C050 Cobaltsuperalloy with TaC 1 18,400 8.0 C) TaC-C056 Cobalt superalloy with TaC1 15,300 7.5 D) TaC-C051 Cobalt superalloy TaC 134,700 16.8 E) TaC-C055Cobalt superalloy TaC 108,600 4.8 F) TaC-C0508 Cobalt superalloy TaC149,000 30.0 G) TaC-CoSOTa Cobalt superalloy Ta & TaC 96,700 5.0

TABLE III Tensile Properties at High Temperature (1832F) Ult.StressComposition Description (psi) Elong.

B) TaC-C050 Cobalt superalloy TaC 25,300 2l.2 C) TaC-C056 Cobaltsuperalloy TaC 31,200 9.4 D) TaC-C051 Cobalt superalloy TaC 33,600 10.2E) TaC-C055 Cobalt superalloy TaC 28,000 10.4 F) TaC-C0503 Cobaltsuperalloy TaC 54,900 7.0 TaC-CoSOTa 25,700 26.0

Cobalt superalloy Ta & TaC

TABL 1V V High Temperature Stress-Rupture Properties Stress(psi)Life(hrs) Composition Description & Temp. to rupture Elong.

A) TaC-Co45 Cobalt superalloy 17,000 276.3 18.7

with TaC (I800F) B) TaC-C050 Cobalt superalloy 17,000 5.42 22.1

with TaC (1960F) C) TaC-C056 Cobalt superalloy 17,000 5.09 10.9

with TaC (2000F) F) Cobalt superalloy 17,000 1000 5 .TaC-CoSOB with TaC(2012F) at 20,000 146 F) Cobalt superalloy 35,000 93.12 5.1 TaC-C0508with C (1832F) Test discontinued no rupture (see explanation below).

The specimen TaCCOSOB was tested at 2,0l2F and 17,000 psi for 1,000hours with no rupture, at which time the stress was increased to 20,000psi. After an additional 146 hours at the higher stress, the test wasterminated to preserve the specimen intact for further examirgtLn FIG. 1is a photomicrograph (250ilof adoiigitudinal section of a cobalt-basesuperalloy with tantalum carbide fibers and having the nominalcomposition F of Table I. In FIG. 1 it may be seen that the tantalumcarbide fibers are relatively long and mutually aligned, 1

thus contributing to maximum strength of the alloy in the direction oftheir alignment. Transmission electron microscopy results show thatthese fibers are cubic and single crystal.

FIG. 2 is a photomicrograph (250X) showing a transverse section of thealloy body of FIG. 1, illustrating pressed as the Larson-Millerparameter, of the same alloy as in FIGS. 1 and 2, with those of Rene 80,a common alloy with relatively high creep resistance. This well knownalloy was selected for the comparison because its creepresistanceproperties, expressed as the Larson-Miller parameter, are known.

The Larson-Miller parameter mentioned above is discussed in the bookHigh-Temperature Materials and Technology by Campbell and Sherwood, JohnWiley and Sons, pages 857-858; and in Transactions ASME, 74, P 7

As may be seen from the curves and data of FIG. 3, the typical nominal(melt) composition F of the tantalum carbide reinforced cobalt-basealloy of the present invention has superior high temperature creepresistance, and thus is capable of achieving a longer life under stressat elevated temperatures. For example, at a temperature of l,832F(1,000C) and 35,000 psi stress, the Rene' 80 alloy of the prior artwithstood the stress for only 2 hours before rupture; whereas under thesame conditions the cobalt-base alloy with tantalum carbide of theinvention withstood the stress for 93 hours before rupture.

At a higher temperature of 2,012F (1,100C) and 17,000 psi, with Renealloy withstood the applied stress for only 5 hours before rupture;whereas under the same conditions the cobalt-base alloy with tantalumcarbide fiber reinforcement (identified as TaC-50B) withstood the stressin excess of 1,146 hours life, i.e., 1,000 hours at 17,000 psi, plus 146hours at 20,000 psi stress. At this point, no rupture had yet takenplace. The results of High Temperature Stress-Rupture tests are listedabove in Table vIV and plotted as Larson- Miller parameters in FIG. 3.It is thus evident that the compositions and TaC fibers have a markedeffect on the stress-rupture properties, particularly at highertemperatures and lower stress levels.

Thus, the alloys of the present invention exhibit a combination ofdesirable properties. As shown in Tables II and III, both at roomtemperature and at elevated temperatures, the unidirectionallysolidified TaC reinforced cobalt-base alloys have good tensile strengthand elongation. They also have good oxidation and corrosion resistanceat high temperatures (not shown); and, as set forth in Table IV, theyhave excellent stressrupture properties at elevated temperatures as highas 2,012F, with ductility in excess of 5 percent total elongation.

It will be obvious to those skilled in the art upon reading theforegoing disclosure that many modifications and alterations in thespecific compositions and microstructures disclosed as non-limitingexamples may be made within the general context of the invention, and

that numerous modifications, alterations and additions v may be madethereto within the true spirit and scope of the invention as set forthin the appended claims.

What we claim as new and desire to secure by Letters Patent of theUnited States is: y

1. An article of manufacture comprising a unidirectionally solidifiedcasting of a nominal composition consisting essentially in weightpercent of about 15.0 percent chromium, 8.5 percent nickel, 6.0 percenttungsten, 18.67 percent tantalum, 1.33 percent carbon,

and the balance cobalt, said casting having the cobaltbase alloy matrixreinforced with a fibrous phase of tantalum monocarbide.

2. A cast article according to claim 1, said fibrous phase forming atleast 12 volume percent of the article.

2. A cast article according to claim 1, said fibrous phase forming atleast 12 volume percent of the article.